The invention relates to a photopolymer formulation comprising matrix polymers, writing monomers and photoinitiators, a process for the preparation of the photopolymer formulation, a photopolymer formulation obtainable by the process, a sheet, a film, a layer, a layer structure or a moulding comprising the photopolymer formulation and the use of the photopolymer formulation for the production of optical elements, in particular for the production of holographic elements and images.
WO 2008/125229 A1 describes photopolymer formulations of the type mentioned at the outset. These comprise polyurethane-based matrix polymers, acrylate-based writing monomers and photoinitiators. In the cured state, the writing monomers and the photoinitiators are embedded with spatial distribution in the polyurethane matrix. The WO document likewise discloses the addition of dibutyl phthalate, a classic plasticizer for industrial plastics, to the photopolymer formulation.
For the uses of photopolymer formulations in the fields of use described below, the refractive index modulation Δn produced by the holographic exposure in the photopolymer plays a decisive role. In the holographic exposure, the interference field comprising signal and reference light beam (in the simplest case, the two plane waves) is formed by the local photopolymerization of, for example, highly refracting acrylates at sites of high intensity in the interference field in a refractive index grating. The refractive index grating in the photopolymer (the hologram) contains all information of the signal light beam. By illuminating the hologram only with the reference light beam, the signal can then be reconstructed again. The strength of the signal thus reconstructed in relation to the strength of the incident reference light is referred to as Diffraction Efficiency, or DE below. In the simplest case of a hologram which forms from the superposition of two plane waves, the DE is obtained from the quotient of the intensity of the light diffracted in the reconstruction and the sum of the intensities of incident reference light and diffracted light. The higher the DE, the more efficient is a hologram with respect to the necessary quantity of reference light, which is necessary for making the signal visible with a fixed brightness. Highly refracting acrylates are capable of producing refractive index gratings with high amplitude between regions with lowest refractive index and regions with highest refractive index and hence permitting holograms having high DE and high Δn in photopolymer formulations. It should be noted that the DE depends on the product of Δn and the photopolymer layer thickness d. The greater the product, the greater is the possible DE (for reflection holograms). The width of the angular range in which the hologram is visible (reconstructed), for example in the case of monochromatic illumination, depends only on the layer thickness d. On illumination of the hologram with, for example, white light, the width of the spectral range which can contribute to the reconstruction of the hologram likewise depends only on the layer thickness d. It is true that the smaller d, the greater the respective acceptance widths. If it is hence intended to produce light and easily visible holograms, a high n·d and a low thickness d are desirable, in particular so that DE is as large as possible. This means that the higher n, the more latitude there is for producing light holograms by adaptation of d and without loss of DE. The optimization of n in the optimization of photopolymer formulations is therefore of outstanding importance (Ref. Hariharan Optical Holography).